Method for constant-current generation and device used to carry out said method

ABSTRACT

The invention relates to a method for constant-current generation by means of a rotational energy source, a generator and a control circuit connected to the generator. Mechanical rotational energy is generated by the energy source and the mechanical rotational energy is transmitted to the generator rotor. The magnetic field distribution produced in the generator is controlled by the control circuit that controls the current distribution in the generator between load current and generator current by feedback control by generating an opposing magnetic field. The power uptake and the current output are adapted without losses and the alternating current generated by the generator is transferred to an averaging direct-current circuit. The invention further relates to a suitable device for carrying out the inventive method.

[0001] The invention relates to a method as claimed in theprecharacterizing clause of patent claim 1 and to a device forgenerating electrical power as claimed in claim 4.

[0002] Electrical power generators, in particular those that produceelectrical power from internal combustion engines, are known, forexample generators whose output frequency has a fixed relationship withthe rotational speed of the drive. Known in particular are electricalpower generators which operate at a constant rotational speed. In thecase of smaller generators having permanent-magnet excitation and a d.c.intermediate circuit, attempts to achieve a constant voltageindependently of rotational speed have usually involved the use ofparallel regulators, in the case of which any excess power generated wasconverted into thermal losses/energy, i.e. was wasted.

[0003] In this regard a combined motor/generator system for generatingelectrical power for single-phase emergency power supply systems hasbeen disclosed, for example, by HONDA, in which an external rotor of agenerator was integrated in an internal combustion engine, with theresult that savings could be made in terms of costly generator shaftsand generator components. This known system was disadvantageous,however, in that it always required a specially designed and adaptedinternal combustion engine as the energy source. This construction alsodoes not satisfy stringent protection class requirements—i.e. it is notsuitable for use in certain environmental conditions, such as outdooruse subject to weathering conditions or else in areas of high humidityor in areas in which a high protection class is required by law, forexample by fire services, technical assistance organizations, fieldhospitals, etc.

[0004] One object of the invention is therefore to specify a method forgenerating electrical power which enables a current level which isindependent of rotational speed for a wide range of input rotationalspeeds.

[0005] The object is achieved according to the invention by a methodhaving the features of patent claim 1.

[0006] Due to the fact that feedback control is now provided in thegenerator itself by varying the distribution of the generated currentbetween load current and feedback current, it is possible to optimizethe generation of electrical power from the rotational energy since, dueto the build-up of opposing magnetic fields in the generator, it ispossible to create very little excess energy that needs to be destroyedthermally.

[0007] The method according to the invention makes it possible for awide variety of rotational energy sources to be used to produce constantcurrent, for example internal combustion engines such as Sterlingengines, diesel engines, Wankel engines and Otto-cycle engines, as wellas turbines, driven wheels, wind-powered wheels, hydraulic motors,pneumatic motors, etc.

[0008] It is advantageous if the rotational energy source, for examplean internal combustion engine, is controlled by means of the controlcircuit. This may be achieved, for example, by adjusting the throttlevalve in a controlled manner, by varying the fuel supply, by adjusting avalve in the case of wheels or turbines driven by water power, byvarying the air/gas supply in the case of gas/pneumatic motors orturbines, by adjusting the turbine blades or the hydraulic pressure orby distributing the volume flow etc. in order to ensure optimum energysupply to the generator. In this case this optimization may be carriedout in terms of fuel consumption, noise, service life, emissions ofpollutants, heat generation, load demands, for example in the case ofdynamic load demands or fluctuations in load, the drive selected (forexample changeover from water/wind power to internal combustion engineor between different motors) [lacuna] avoiding critical rotational speedranges or load ranges. In this manner it is possible, particularly inthe case of diesel engines, to operate at defined base loads in order toavoid incomplete combustion and resultant deposits in the combustionchamber as well as emissions, or else to provide specifically“clean-burn cycles” for carbon filters in order to avoid the need formaintenance work.

[0009] It is particularly advantageous if the permanent-magnetarrangement of the generator rotor being driven and the stator assemblyare geometrically matched to one another so that, in combination, thesegeometries have both local magnetic saturation effects and specificfield-distortion effects when the generator is in operation, with theresult that a constant current is generated that is independent of therotational speed. Here, a feedback controller, connected to thegenerator, controls the magnetic field induced in the generator inaccordance with the generator parameters such that an essentiallyconstant current level is output by said generator, independently of theenergy introduced into the generator and drawn off. It is possible tocalculate the appropriate configuration for the geometries in the mannerknown to those skilled in the art, as is described, for example, by E.Spring in “Elektrische Maschinen” [Electrical machines], Springer VerlagBerlin/Heidelberg 1999 as well as [lacuna] the finite element method.

[0010] It is thus possible in a simple manner to use even energy sourceswhose behavior cannot be controlled, such as wind-driven orhydroelectric power stations, as rotational energy sources and to usethem to obtain a constant current without too much difficulty.

[0011] It is advantageous if the generator has an external rotor havingpermanent magnets, since then the physical size may be kept small and,with the same mass, the mass moment of inertia is considerably greaterthan in the case of the conventional internal rotor configuration, as aresult of which flywheels on motors generating rotational energy can bereduced in size or dispensed with entirely. By this means it is alsopossible for the fixing of permanent magnets on the rotor to besimplified or for the load on said permanent magnets brought about bythe centrifugal force to be used to advantage for reinforcing theconnection. It is particularly advantageous in this case if the externalrotor is designed to be integral with an outer housing which protectsthe generator, has additional flywheel properties and providesencapsulation against the ingress of moisture and dust, or otherimpurities. This outer housing may advantageously also be fit with finsor the like which provide cooling, if necessary, for the generatorduring operation without—as is often the case in the prior art—the needfor the provision of additional cooling devices. This design alsocontributes to a compact structure, high reliability of the electricalpower generator and cost-effective manufacture.

[0012] In one particularly preferred embodiment, the rotor is connecteddirectly to the output drive shaft of the energy source—preferably amotor. This is made possible by the electrical power being generatedindependently of the rotational speed and has the advantage, inter alia,that savings can be made in terms of bearings, couplings, etc., as werenecessary in the case of known electrical power generators, and that aneasier and simplified construction is achieved. The number of partssubject to wear can also be reduced by this means.

[0013] Preferred exemplary embodiments of the invention are explained inmore detail below with the aid of the attached drawing, in which:

[0014]FIG. 1 shows a flowchart for the method according to the invention

[0015]FIG. 1a shows a circuit diagram for a feedback controller whichimplements the method according to the invention

[0016]FIG. 2a shows a part of a section through an optimizedrotor/stator arrangement of an electrical power generator according tothe invention having ferrite magnets in the rotor

[0017]FIG. 2b shows a partial view of a section of an optimizedrotor/stator arrangement of an electrical power generator according tothe invention having NdFeB magnets in the rotor

[0018]FIG. 2c shows a partial view of a section of an optimizedrotor/stator arrangement of an electrical power generator according tothe invention having NdFeB magnets

[0019]FIG. 3 shows an illustration of one rotor geometry and

[0020]FIG. 4 shows an illustration of a further preferred rotorgeometry.

[0021] As shown in FIG. 1, the method according to the inventionprovides a feedback controller which is capable of varying the ratiobetween a load current and the generator current by means of aregulator. The current generated by the generator in the stator 12 isthen measured and is checked by internal logic in a feedback controllerto ascertain whether it is within the desired range. If the generatedcurrent is not in the desired range, the load current/generator currentratio is varied by the feedback controller so that the generator currentis in the desired value range again. The generator current/load currentratio is constantly ascertained and adjusted in this manner in order toensure that the generator current remains constant. A current ispreferably used as the load current which induces an opposing magneticfield in the generator, with the result that it is then possible for theamount of current generated to be reduced.

[0022]FIG. 1a shows one preferred embodiment of a control circuit forcarrying out the method in FIG. 1. The generator G is in this case usedas a current source, as opposed to the prior art in which generators areusually used as a voltage source, and can be controlled by feedback. Thecurrent output by said generator G is rectified by means of theuncontrolled 6-pulse rectifier in the circuit, designated BGU, and isfed to the intermediate circuit control system.

[0023] The generator is then short-circuited if it generates too muchcurrent, so that no energy is output and the voltage can be maintainedat the desired value. The controlled switching elements RWS1 and RWS2(for example IGBPs=isolated gated bipolar transistors) can be actuatedand closed separately from one another, with the result that voltage isbuilt up symmetrically between the center of the switches and theterminals. In the process, the switching element opposite, in FIG. 1a,charges the diagonally opposite capacitor, i.e. RWS1 for C2 and RWS2 forC1. If both the IGBPs are open, both the capacitors C1 and C2 arecharged. If both are closed, the generator is short-circuited and nocharging of the capacitors takes place. If only one IGBP is open, theobliquely opposite capacitor is charged. Therefore, at low generatorrotational speeds, “current pumping” via the capacitors will still leadto a desired end voltage. In this manner the drive motor is notsubjected to heavy or excess loads by the generator, since the methodaccording to the invention distributes the current such that the ratedvoltage is represented even at low generator rotational speeds, i.e. thegenerator becomes more tolerant in terms of rotational speed. Forcomparison purposes: A “normal” generator having a rating ofapproximately 10 kW and the same outer geometry has a rotational speedtolerance of approximately 30% at—for a constant voltage design—about1:1.3. In contrast to this, a tolerance of about 1:3.5 is achievedaccording to the invention. The generator according to the invention is,therefore, a constant current generator which permits torque to beapplied to the drive shaft and can tolerate, without difficulty, dynamicchanges in rotational speed over a relatively wide range of rotationalspeeds without overvoltages occurring. In many cases it is possible inthis way for even relatively small motors to be used for the desiredvoltage generation.

[0024]FIG. 2 shows preferred geometries for the rotor/stator arrangementof an electrical power generator according to the invention in which,for simplification purposes, the wire windings around the cores of thestator 12 have been omitted. In this case, as can be seen from thedrawing in FIG. 2a, for example, more than two coil cores of definedgeometry are positioned opposite a magnet 11 in the rotor 10 in the caseof ferrite permanent magnets for correspondingly forming an inducedmagnetic field which results in a constant current during rotation. FIG.2b shows an arrangement having an NdFeB magnet—a slightly differentmagnet geometry has been used here, as well as a slightly differentstator geometry 12.

[0025] The particular geometry must be optimized according to therequirements of the individual case on the basis of the differentpermanent-magnet materials or the different behavior of the magneticfields, as is known to those skilled in the art.

[0026] The geometrical configuration brings about a self-regulatingeffect in the generator itself—i.e. an ever increasing braking effectwill be observed at higher rotational speeds due to the dynamic build-upof the field in the magnetic material, which does not take place atlower speeds. As the rotational speed increases, the magneticallyeffective pole area is reduced by the field-distortion effect, producinga partial local field short-circuit in the extended air-gap region(air-gap exit zone). The rotor geometry has specific zones for localmagnetic field saturation.

[0027]FIG. 3 represents an embodiment of the rotor which also serves asa protective and enclosing housing in this case. As can be seen from thedrawing, in this and similar embodiments the internal, sensitive stator12 and its windings are protected by the pot-like configuration of therotor 10, while gas guide vanes 14 and other air-guiding devices, suchas grooves etc. can be provided on those rotor surfaces which do nothave permanent magnets 11 fit to them, as a result of which heat can bedissipated from the generator, or air can be passed through it, withoutany difficulty. However, the pot-like configuration is not essential. Itis only important that the rotor configuration reliably maintains thenecessary geometric arrangement of the permanent magnets fit to it. Ifbetter cooling is required, it is possible for apertures or else otherguide sections to be provided in the rotor 10.

[0028]FIG. 4 shows an exploded illustration of an alternative embodimentfor the rotor design. In this case, the rotor 2730 is formed separatelyfrom an outer housing 4021, as a result of which the rotor is providedwith better protection. The important feature here is the drivingelement 2710, which makes it possible for the rotor to be suspended atone end and acts at the same time as a radial displacement fan.

[0029] The rotor may be produced from any suitable material—it may bemade of metal, but also at least partially—with the exception of thepermanent magnets—of other materials, for example plastic, ifappropriate reinforced plastic which has the advantage of being lesssusceptible to corrosion and low in weight, or else a suitable ceramic.

[0030] Of course, the invention is not limited to the embodimentsillustrated, rather there are many possible refinements of the teachingof the invention within the protective scope of the claims, which areobvious to those skilled in the art.

1. A method for generating constant current by means of a rotationalenergy source, a generator and a control circuit connected to thegenerator, comprising: production of mechanical rotational energy by theenergy source, transfer of the mechanical rotational energy to thegenerator rotor, control of the magnetic field distribution generated inthe generator by means of the control circuit which is capable ofregulating, by feedback control, the current distribution in thegenerator between the load current and the generator current by anopposing magnetic field being induced in the generator such that thepower consumption is matched to the current output with low losses andthe alternating current generated by the generator being drawn off andfed to an averaging d.c. circuit.
 2. The method as claimed in claim 1,characterized in that the generator rotor is an external rotor.
 3. Themethod as claimed in claim 1, characterized in that an internalcombustion engine, a turbine, a driven wheel, a wind-powered wheel, ahydraulic motor, a pneumatic motor or the like is provided as therotational energy source.
 4. The method as claimed in one of thepreceding claims, characterized in that the rotational energy source iscontrolled by means of the control circuit.
 5. A device for producingelectrical power, characterized by: a rotational energy source, agenerator having a driven generator rotor (10) with a specific geometryfor the magnetic field and a stator assembly (12) having a geometrywhich is matched to the generator rotor, said geometries being selectedsuch that they have both local magnetic saturation effects and specificfield-distortion effects in order to produce a constant current which isindependent of the rotational speed, and a feedback controller,connected to the generator, which controls the magnetic field induced inthe generator in accordance with the generator parameters such that anessentially constant current level is output by said generator,independently of the energy introduced into the generator and drawn off.6. The device as claimed in claim 5, characterized in that the rotor(10) is an external rotor.
 7. The device as claimed in claim 5 or 6,characterized in that the external rotor (10) is designed to be integralwith the outer housing which has air guide vanes (14) on its outercircumference.
 8. The device as claimed in claim 5 or 6, characterizedin that a rotating part of the energy source, such as the output driveshaft or a flywheel, is directly connected to the rotor (10).